The present invention relates to an optical fiber collimator including a microlens array having a plurality of microlens elements formed in the surface of a transparent substrate.
FIG. 12 shows an optical fiber collimator 20 of the prior art. The optical fiber collimator 20 includes a planar microlens array 21 and an optical fiber array 22, which includes a plurality of single mode optical fibers 22A. The planar microlens array 21 includes a plurality of semispherical, gradient index microlens elements 24, which are formed on one surface 23a of the glass substrate 23. The diameter of each microlens element is about several tens of micrometers to several hundreds of micrometers. Japanese Laid-Open Patent Publication No. 57-53702 describes manufacturing the planar microlens array 21 by performing ion exchange. A metal mask is first applied to one surface 23a of the glass substrate 23. Photolithography is then performed to pattern the metal mask and form a plurality of mask openings in the metal mask. The glass substrate 23 is then immersed in molten salt, which contains ions of elements having the effect of increasing the refractive index. Ion exchange reaction occurs between the molten salt and the glass of the glass substrate 23 exposed from the mask openings. This forms a plurality of gradient index regions, or the microlens elements 24, in the surface 23a of the glass substrate 23.
In the planar microlens array, the array accuracy of the microlens elements mainly depends on the formation accuracy of the mask openings in the metal mask. The formation accuracy is extremely high. Thus, the planar microlens array is widely used in the optical communication field, the OA equipment field, which includes copy machines and facsimile machines, and the medical field, which includes endoscopes.
Japanese Laid-Open Patent Publication No. 2001-305376 describes an optical cross connect module, which is a combination of an optical switch array and an optical fiber array. The optical cross connect module is used in the optical communication field to switch the transfer designation of a plurality of optical signals. The optical cross connect module normally includes an optical fiber collimator. The optical fiber collimator is manufactured by optically coupling the planar microlens array and the optical fiber array. Accordingly, the planar microlens array is an important optical element of the optical fiber collimator.
Japanese Laid-Open Patent Publication No. 2000-304966 describes a planar microlens formed by adhering a plurality of glass substrates to each other.
In the prior art optical fiber collimator 20 of FIG. 12, the microlens elements 24 are formed in only one surface 23a of the glass substrate 23. This optical fiber collimator 20 has the problem described below.
It is difficult to manufacture the planar microlens array 21 with the microlens elements 24 having a sufficient numerical aperture (NA). For example, when forming an optical fiber collimator array for collimating the light emitted from the single mode optical fibers, if the planar microlens array 21 includes microlens elements 24 having a low numerical aperture, the insertion loss is large in comparison to when using lenses having a relatively high numerical aperture, such as gradient index rod lenses. Thus, it becomes difficult to obtain low insertion loss as is normally required in optical communication applications. When using microlens elements 24 having a low numerical aperture, in the emission light of a single mode optical fiber, only the light in a relatively narrow angular range can be used effectively. Further, the employment of the peripheral portion of a lens, where the aberration is unsatisfactory, is not preferable since this would decrease the light transmission accuracy.